One belief is that McDonald's uses cow eyeballs in its products, permitting it to brand them as "100% beef".[4] However, the USDA mandates that all beef by-products, including cow eyeballs, be appropriately labeled. McDonald's, has asserted that its products contain "100% pure USDA inspected beef; no additives, no fillers, no extenders." In addition, cow eyeballs are actually more expensive than the more commonly eaten cow parts, due to demand from scientific institutions for experiments.[5]

A related claim is that McDonald's buys its meat from a company called "100% beef", making it possible for McDonald's to call beef by-products and soy products "100% beef".[6]

Around March–April, 2000, an Internet rumor spread via e-mail in Brazil claimed that McDonald's meat was actually made from a genetically modified animal maintained in a laboratory. The e-mail stated that "the few who saw it assure it is a very unpleasant sight: they have no limbs or horns, no bones (undeveloped cartilage instead), no eyes, no tail and no fur; its head is about the size of a Baseball; they are fed through tubes connected directly into their stomach".[7]

The e-mail carries on saying that "some irreversible health damage can be done by eating this meat, resulting in diseases who manifest themselves in a way similar to AIDS, and have symptoms related to Alzheimer's Disease" and ends encouraging the reader to boycott McDonald's until it sells actual beef. The urban legend has also been attributed to other fast-food chains and animal products, such as KFC and mutant chickens.

This rumor is that McDonald's uses pig fat in its milkshakes, ice cream, and fried potatoes. McDonald's provides complete ingredient lists for all of its products on each of its regional websites: this includes unidentified fats within the ice cream used to make soft serve cones and sundaes.[8][9] McDonald's Australia, however, specifically mentions that "there is definitely no lard or pig fat in the McDonald's Soft Serve."[10] This rumor should not be confused with the fact that McDonald's has in the past used beef tallow as its frying oil.[11]